1. Field of the Invention
This invention relates to a composition and method for greatly increasing the divalent ion tolerance of silicate gelling systems. More specifically, the method and composition of the present invention greatly increases the effectiveness of such gelling systems employed in petroleum reservoirs.
2. Description of the Prior Art
In oil field operations the production of excess quantities of water causes major economic loss due to decreased oil production, increased lifting costs, and increased separation and disposal costs associated with large amounts of produced water. Other production-related problems include increased corrosion rates and an increased tendency for emulsion and scale formation. Therefore, there is a critical need to reduce excessive water production.
Gelling polymers for water shutoff in petroleum reservoirs have been extensively evaluated. Based on such evaluations, the sodium silicate gel system has been identified as providing significant advantages for use in water shutoff. These advantages include significantly lower cost, better control over gel time, environmental safety, high temperature stability and stability of the gel components during shipment and storage. However, silicate gels also have an important disadvantage. Specifically they are sensitive to high calcium and magnesium concentrations.
U.S. Pat. No. 4,640,361 discloses that the addition of divalent ions, such as calcium or magnesium ion, to sodium silicate solutions will cause immediate gelation of the solution. However, this is highly undesirable in treating well formations, because many formations contain brines that are high in divalent ion concentration. As the sodium silicate solution comes in contact with the formation brine, premature gelling will result that prevents the sodium silicate solution from completely entering the formation.
Aqueous silicate solutions that are gelled or solidified with acid-producing activators are well-known and have been used in various applications, including grouting processes and in sealing and consolidation processes in subterranean well formations. Some examples of such applications are disclosed in U.S. Pat. Nos. 3,202,214; 3,375,872 and 3,435,899.
In alkaline solution, sodium silicate contains various molecular weight silica polymers. When sodium silicate is acidified to a pH value of less than 10.6, a three-dimensional network of silica polymers forms that creates a gel. A slight decrease in the pH greatly reduces gel time. As a result, gel times are difficult to control by the addition of acid.
Thermally responsive sodium silicate solutions employ an activator that results in gelation only after the solution is heated. Thermally responsive activators are disclosed in U.S. Pat. Nos. 4,293,440; 4,384,894; 4,640,361 and 5,320,171.
Gels have been used as blocking and diverting agents to treat injection and production wells for more than five decades (U.S. Pat. No. 2,402,588). Several factors determine the success of a gel treatment in the field, including candidate selection, identification of the source water, proper choice of the gelling system and placement of the gel into the target zone.
U.S. Pat. No. 4,137,087 employs alkali silicates, metal ions from the group Al3+, Fe3+, Cr3+and Sn2+, and ligands from the group consisting of ethylenediaminetetraacetate, diethylenetriaminepentaacetate, N-(hydroxyethyl)ethylenediaminetetraacetate, nitrilotriacetate, and 1-3-propanediaminetetraacetate, which cures the silicate solution by evaporation. There is no disclosure of temperature activated gelation in U.S. Pat. No. 4,137,087.
U.S. Pat. No. 4,732,213 employs a chelating agent and a divalent ion such as calcium ion, to form a latent gelling agent. However, this patent is directed to colloidal silica gels, not sodium silicate gels. There are significant and distinct differences in the behavior of colloidal silica gels versus sodium silicate gels.
It is, accordingly, an object of the present invention to provide a composition and a method for gelling aqueous silicate compositions by using thermally responsive gelation activators which reduce the production of excess quantities of water, reduce corrosion rates and decrease the tendency for emulsion and scale formation. This gelling composition exhibits divalent ion tolerance.
The present invention contemplates the treatment of subterranean well formations to plug or seal a zone in said formation with an aqueous silicate solution containing an alkali metal silicate, at least one activator and a mixture of a chelating agent and a phosphonate to achieve the desired thermal responsiveness and produce the desired tolerance to divalent ions.
It has been found that the combined or joint use of a chelating agent and a phosphonate serves to increase the divalent ion tolerance of the silicate solutions. In other words, to inhibit the rate of gel formation or premature gelation, to a much greater extent than that which occurs when either chelating agents or phosphonates are used separately or individually.
By means of the composition and method of the present invention improved control over the gel time of silicate solutions is achieved in the presence of divalent ions. In addition, a high level of temperature stability is maintained, as well as stability of the gel components during shipment and storage.
The silicates which can be employed in the composition and process of the present invention are the water-soluble silicates which form silicate polymer chains or which form a gel upon acidification. The preferred silicates are those of the alkali metals, particularly sodium, potassium, and combinations thereof. These silicates are commercially available as dry powders, or as concentrated aqueous solutions containing 38 to 55 parts solids per 100 parts of solution, and having a pH in the range of from about 10 to about 13.
In treating subterranean well formations in accordance with the present invention, the aqueous silicate solution is mixed with one or more activators to achieve the desired thermal responsiveness. Any of the activators disclosed in U.S. Pat. Nos. 4,293,440, 4,384,894, 4,640,31 and 5,320,171, the specifications of which are incorporated herein by reference, can be employed. Exemplary activators are sodium trichloroacetate, trichloroacetic acid, ethyl trichloroacetate, ethylacetate, hydrochloric acid, sodium trichloroacetate, trichloroacetic acid, ethyl trichloroacetate, mannitol, glycerol, sucrose, lactose, dextrose, fructose, galactose, mannose, maltose, xylose, and urea, with urea being preferred.
The aqueous silicate solution must also contain a mixture of at least one chelating agent and at least one phosphonate to produce the desired tolerance to divalent ions to avoid premature gelation.
Exemplary of the chelating agents which can be employed in accordance with the present invention include propanediaminetetraacetate, diethylenetriaminepentaacetate, N-hydroxyethylenediamine triacetic acid, hydroxyethyliminodiacetic acid, nitrilotriacetate, and 1-3-propanediaminetetraacetate. A concentration of from about 0.5% to about 3.0%, by weight, of chelating agent has been found to be effective in the present process.
Exemplary of the phosphonates which can be employed in conjunction with the chelating agents are sodium phosphonate, phosphonic acid, diethylenetriamine pentamethylene phophonic acid, methylenephosphonate, as well as their derivatives. A phosphonate concentration of about 40 ppm to about 200 ppm has been found to be effective in the process of the present invention.
In accordance with the composition and process of the present invention, gelation occurs at a temperature between about 100xc2x0 F. to about 350xc2x0 F., preferably between about 120xc2x0 F. to about 200xc2x0 F.